One very useful skill for dealing with electronic
systems is being able to identify capacitors and safely discharge capacitors. You can see here
I have a bunch of different type of capacitors. So electrolytics, ceramic... the thing with
all these capacitors right here in particular, these five, is that none of these are dangerous.
If they're fully charged it's not dangerous to touch these, but the looks on these can be
deceiving. This is especially true when it comes to electrolytic capacitors, which are these guys
here. For example if I showed you this capacitor and this capacitor, which one of these
two do you think is more dangerous? Well if you guessed this capacitor, you're right.
This capacitor right here is rated for 400 volts, 68 microfarad. This one is rated 35 volts, 10,000
micro farad. So even though this holds a lot more charge, it doesn't hold the voltage required
to electrically shock you, whereas 400 volts is plenty enough to shock you. It probably wouldn't
be a serious shock but it wouldn't be pleasant, and it can get even more deceiving.
For example, if I bring this one in. Now, you might be thinking this one obviously
looks dangerous, but this is actually rated for just 25 volts. So if this was fully
charged, and you touch these terminals, you wouldn't even feel it. This one on the
other hand, even though it's physically smaller, this one is more dangerous. The sense that
it's rated for 200 volts at 1000 microfarad. So this one could give you a pretty
substantial shock if it's fully charged. Even more confusing, a capacitor like
this one which is an x2 capacitor, this one's rated for 275 volts yet it's
physically smaller than even this one. When you where you start getting into the really
dangerous capacitors they start looking like this, and you can see here I have the two terminals
shorted out permanently. This is actually a capacitor from a microwave oven and this
capacitor is rated for two kilovolts. So the truly dangerous thing about these is
not only can it potentially shock you and cause burn damage, or nerve damage, or
something like that... if you touched the terminal of this with one finger and
the other terminal with the other finger, well it's going to send 2100 volts through your
chest, at that point, and potentially through your heart. Now one feature of these very large
capacitors, usually, is you can see this symbol right here it has a capacitor and a resistor
on top of it. What that means is that inside of this can there's a capacitor, and then there's
a resistor shorting out the two leads. So when this is removed from power, it will start slowly
discharging itself to make it relatively safe. The reason I have it shorted out, though, and
generally the reason you want to keep these shorted out, is you never really want to rely on
just one point of failure. So if that resistor fails for any reason, this capacitor could
potentially store quite a large amount of power and you wouldn't be one any the wiser, but while
these capacitors have safety mechanisms built in, these ones don't. So there is no resistor inside
of here. So if you charge this capacitor up, it's going to maintain it's going to
have some leakage, and it's going to start discharging slowly over time. It will
maintain a significant amount of charge even when disconnected from power. So one thing
to kind of help you understand how capacitors work is to think of capacitors as
sort of like electrical springs, and like a spring normally when there's no load on
it there's no real danger from it at all. If you put a load on a spring, let's say you stretch
out the spring, that would be the equivalent of charging up this capacitor, for example.The
issue with this is, think about what happens if you take a spring that's been pulled out as far
as it can and then let it go. Well it's going to try and release all of its energy as fast as
possible, and usually with pretty violent results. The same is true with capacitors. For example,
if you think of the capacitor like a spring, if we were to take us like a screwdriver like
this and short out the two terminals like this, that would be the equivalent of taking our spring
and just letting it go. So it's going to try and release all of its energy all at once, and to
demonstrate this, what i can do is i'll take this 35 volt 10 000 microfarad capacitor, and short
out the two leads. So you can see what happens. This is 35 volts so it's safe to touch
even when fully charged, but it will still have pretty explosive results if you try and
discharge it. So let me charge up to 30 volts All right now that's charged I can take these off. Now take my screwdriver. Again, this is
like taking the spring just letting it go. So very quickly tried to release all of its energy
onto my poor screwdriver here, which you can see is a little bit blackened up. Now if you promise
not to try this at home, I'll do this test again, this time with a 400 volt capacitor, and what I'll
do is I'll be charging it up to about 300 volts using this transformer and rectifier. So I have
my meter here also to keep track of the voltage. So I'll turn the supply on real quick. So about 250 volts on that capacitor, and
this time, though, if you touched it with your bare finger it would shock you. So I'm
just going to use my screwdriver, keeping a good bit of distance, yep. So you probably
don't want to have that happen to your finger. So you can discharge a capacitor like this. Like
I said though it's pretty much like releasing a spring under full tension. Not only can
it, you know, obviously cause damage to your tools like this here, but it could
even cause damage to the capacitor itself. This capacitor may not be very
happy anymore that I did that. So generally a better way to discharge capacitors
is to use some type of resistor in place. So this right here is a 10 watt 10 ohm resistor and
this is a 100 ohm 5 watt resistor, and these are ceramic resistors. So they can get pretty hot.
You can also use larger resistors like this one, although you have to keep in mind that
discharging a capacitor this way is really only safe when you're using it on one that is not
physically able to shock you like this one here. So I have my oscilloscope set up here and I'll
show you the differences in voltage drop between if I just do a direct short and if I hook up a
resistor like this. Okay, so I'll go ahead and charge up the capacitor up to 30 volts. Now I'll
take my screwdriver and short out the two leads, and you can see on the oscilloscope that
the voltage dropped almost immediately. So this time I'll do the same test but I'll use a
resistor to discharge the capacitor instead of just my poor screwdriver here. So I'll
go ahead and charge the capacitor up, and it's fully charged. So now I will
apply the resistor to the leads there, and you can see the voltage drop
was much more steady this time. Let's try it again just to emphasize it. Now you can see the voltage drops off very very
gradually, no sparks came out of this. So the main takeaways from this video are to always
pay attention to the labels on capacitors, especially if you see large ones. The label
really tells you what the capacitor will be like like. For example, this one is not very
dangerous, but this one is potentially dangerous even though it's smaller, and when you're going
to discharge your capacitor generally it's a better idea to use a resistor. You want to make
sure that when you do this, you're only doing it on a capacitor that is safe to touch. If you're
dealing with a larger capacitor like one of these, I would suggest not trying to discharge
this. I would suggest not even working on the equipment if you don't feel comfortable
with it just to avoid injury. Find someone who is comfortable dealing with high voltage
and knows how to handle large capacitors, but most importantly, if you see a capacitor
like this one, be extra careful because this one will hurt you pretty badly if it is in some
way defective or has not discharged properly.
